Manchester code receiver

ABSTRACT

An improved Manchester code receiver is disclosed which samples the received signal and subtracts from that sample a previous sample of the received signal delayed by a half-bit time interval. A timing extractor selects sample timing from the central zero crossing of the received signal. The sample time is selected to be a quarter-bit time after the zero crossing time of the received signal.

This application is a continuation-in-part of application Ser. No.044,267, filed Apr. 30, 1987, now abandoned.

FIELD OF THE INVENTION

This invention relates to data receiver circuits and, more particularly,to a receiver for and method of recovering a Manchester encoded signal.

BACKGROUND OF THE INVENTION

Manchester coding has long been popular for low-cost short distance datacommunication applications despite its inefficient use of bandwidth. Inone application, Manchester coding is used in some local area networksto provide data transmission over an unshielded twisted pair cable. Thecoder's most desirable property is the presence of a zero crossing inthe center of every bit period. This property permits both low-costclock timing recovery and very fast start-up from a no signal condition.The latter benefit is particularly important in multiple userconfigurations such as local area networks.

Commercially available Manchester receivers operate by sampling thereceived signal at either one-quarter bit before or after mid-bit timeperiod. Because of the popularity of Manchester coding for datatransmission, there is a continuing need to improve the signal-to-noise(S/N) ratio of the received signal and reduced intersymbol interference.

SUMMARY OF THE INVENTION

In accordance with the apparatus and method of operation of the presentinvention, an improved Manchester code receiver operation results bysampling the received signal and subtracting from that sample a previoussample of that signal delayed by a half-bit time (0.5T) interval. Atiming extractor selects timing from the central zero crossing of thereceived signal. A sampler circuit samples the input signal at theselected signal zero crossing time plus 0.25T seconds. The resultingManchester code receiver offers improved performance in the presence ofnoise and linear distortions.

BRIEF DESCRIPTION OF THE DRAWING

In the drawings,

FIG. 1 shows a block diagram of a Manchester code receiver in accordancewith the present invention;

FIG. 2 shows a typical Manchester encoded signal;

FIG. 3 shows a generalized band-limited Manchester encoded signal;

FIG. 4 shows an input signal s(t) to the disclosed Manchester codereceiver which is received over a linear channel or a facility inresponse to a generalized band-limited Manchester encoded signalinputted thereto;

FIG. 5 shows a combined signal generated by a delay and subtractionoperations; and P FIG. 6 shows an "eye" pattern outline for the signalof FIG. 5;

FIG. 7 shows another embodiment of the Manchester code receiver inaccordance with the present invention;

FIG. 8 shows a method of obtaining the central zero crossing time of asampled input signal; and

FIG. 9 shows an embodiment of the Manchester code receiver of FIG. 7with a performance monitor circuit added thereto.

DETAILED DESCRIPTION

Referring now to FIG. 1, there is shown a block diagram of a Manchestercode receiver in accordance with the present invention. The inputterminal 100 receives a Manchester encoded signal from a digitalcommunication facility (wire, cable, etc.), magnetic recording source(disk, tape, etc.), or other source. If an optical signal is receivedover a fiber optic facility, an optical converter is needed to convertthe Manchester encoded optical signal into an electrical signal forinput to the disclosed receiver. Alternatively, the present inventionmay be implemented using optical components which perform opticalfunctions in a manner equivalent to the electrical functions performedby the components of the disclosed receiver.

Shown in FIG. 2 is a representation of a typical transmitted Manchesterencoded signal. Transmission of this signal or its negative is the mostcommon form of Manchester signaling. This signal achieves binarycommunication at a bit rate of 1/T.

The Fourier transform of the waveform shown in FIG. 2 is: ##EQU1## Thefirst null of (1) is ω=4π/T, which is four times the Nyquist rate for abasic binary system of rate 1/T. This is indeed exorbitant use of systembandwidth.

Bandwidth occupancy can be improved somewhat by the use of standardpulse shaping techniques at the transmitter. While no transmit signalfiltering is required for use with the present receiver, any type oftransmit signal filtering may be utilized without modification to thepresent receiver.

In FIG. 3 a well-known band-limited Manchester signal is generated bypassing a pair of impulses 301 through a linear filter H(ω) 302, whichis the Nyquist filter that would be used by an encodednon-return-to-zero system operating at rate 2/T. The spectral bandwidthis now only twice that of a standard data transmission channel, such asthe commonly used voice grade channels. Note, the analysis and figuresdescribed in the remainder of this specification use the typical for ofManchester signal shown in FIG. 2, transmitted over a wire pair, whichis a linear transmission system. Because the band-limited Manchestersignal 301 and linear filter 302 of FIG. 3 may be considered as anapproximate equivalent of the typical Manchester signal shown in FIG. 2,the analysis and figures would likewise be applicable thereto.

The amplitude spectrum of the band-limited form of the Manchester signalis ##EQU2## Because of the Nyquist property of H(ω),

    f(t)=0, t=(n±1/4)T, n=±1, ±2,                     (3)

If H(ω) is real, F(ω) is pure imaginary, f(t) is odd, then f(0)=0,preserving the mid-bit zero crossing that is the Manchester code'simportant characteristic.

Equation 3 states that the signal may be sampled at either one-quarterbit before or after mid-bit time period without intersymbolinterference. In accordance with the present invention, the signalsample at both of these instants are utilized by using a matched filterin the receiver to obtain optimum signal detection.

With the pulse pair of FIG. 3, if the transmit filter, the transmissionchannel and the receive filter form a linear system with overall impulserespose g(t), then the received signal presented to the input terminal100 of a Manchesterreceiver is ##EQU3## where α_(n) is the binaryinformation sequence and n(t) is an additive filtered noise. A typicalrepresentation of signal s(t), after transmission over a wire pair, isillustrated in FIG. 4 for a single Manchester signal shown in FIG. 2having a 10 megabit data rate, i.e., T=0.1 microseconds (μsec).

The origin is conveniently chosen at the epoch where E[α_(o) s(t)]=0,which implies that

    g(T/4)=g(-T/4).                                            (5)

With this choice of time origin, the α_(o) is detected as the sign of##EQU4## where

    h.sub.r (t)=g(t)-g(t+T/2)                                  (6b)

or as the sign of ##EQU5## where

    h.sub.l (t)=g(t)-g(t-T/2).                                 (7b)

The "eye" pattern (not illustrated) of the received signal s(t) as seenon an oscilloscope would contain two eyes per bit interval. Equation(6a) corresponds to sampling the right eye, while equation (7a)corresponds to sampling the left eye.

The following paragraphs describe how the present improved Manchestercode receiver detects the received signal s(t). In the followingdescription, each item of each figure has a reference designationassociated therewith, the first number of which refers to the figure inwhich that item is first mentioned (e.g., 110 is located in FIG. 1).

The following description makes joint reference to FIGS. 1, 3, 5 and 6.In accordcance with the present invention, an improved ManchesfterreceiverFIG. 1 results when sampler 101 obtains a sample of the receivedsignal s(t), FIG. 4, which is delayed (102) by a half-bit interval (T/2)and subtracted (103) from a current sample of the received signal s(t).This approach represents a matched filter approach to detection of thereceived signal s(t).

Although sampling 101 occurs before the delay 102 and subtract 103operation in the present invention, it it mathematically convenient toconsider a signal y(t) that would result from subtracting a replica ofthe input signal, delayed by T/2, from the received signal. Thiscombined signal y(t) is shown in FIG. 5. The corresponding eye patternis shown in FIG. 6. It should be noted that sampling of y(t) wouldproduce the same values that occur at the output of the subtracter inthe present invention.

In FIG. 1, the received signal 100 is sampled by sampler 101 at timeintervals spaced by half the bit interval, T/2. The timing instants aregoverned by timing extractor circuit 104, which detects zero crossing ofthe received signal 100 and which controls the sampler 101 samplingtimes so as to occur T/4 before and after the central zero crossing.

Delay circuit 102 delays the output of sampler 101 by T/2 and subtractercircuit 103 subtracts this delayed sample from the next output of thesampler. Decision circuit 105 generates the output data signal 107 so asto agree with the sign of the output 106 of the subtracter 103. Thedecision circuit 105 is pulsed by the output of the timing extractor 104so as to generate a new binary output 107 once during each bit intervalT.

The equation for the output 106 of the subractor 103 is:

    y(T/4)=1/2[s(T/4)-s(-T/4)]                                 (8)

and the output data 107 is generated to agree with the sign of y(T/4).The factor of 1/2 is strictly for mathematical normalization, and is notneeded in an actual implementation.

The receiver circuits may be implemented using either well-known analogsample circuits or well-known digital circuits. In an analogimplementation, sampler 101 generates an analog output sample equal tothe value of its input signal at the time instant of the output of thetiming extractor. The delay circuit 102 may be a well-known sample andhold circuit, which retains an analog value until the following samplinginstant. The subtracter 103 can also be a well-known device to befollowed by a decision circuit 105 which can be implemented using awell-known threshold circuit.

Alternatively, the received sample can be converted to a sequence ofdigital bits using any well-known analog-to-digital conversion circuit.The delay can then be implemented to include digital storage.Subtraction can be implemented using well-known digital arithmeticcircuits, and the decision circuit would then consist of extracting themost significant bit of the subtraction result.

Clearly, any mix of the above-described analog and digital techniques,and well-known variations of them, can be used to implement the presentinvention.

The technique subtracting a delayed sample of the received signal 100from a current sample is equivalent to sampling the original input bothbefore and after the central zero crossing, and choosing the samplewhich is larger in magnitude. This leads to less noise and intersymbolinterference than using either one of the input samples alone.

NOISE INTERFERENCE

In the absence of intersymbol interference, the received samples in thebasic detector are

    s(±T/4)=±α.sub.o g(0)+n(±T/4).              (9)

In either case, the signal to noise power ratio at the sample is##EQU6##

For the modified receiver under the same conditions,

    y(T/4)=1/2α.sub.o g(0)+n(T/4)+α.sub.o g(0)-n(-T/4)=α.sub.o g(0)+1/2n(T/4)-n(-T/4)         (11)

The signal to noise ratio in this case is ##EQU7## Letting R_(n) (τ) bethe normalized autocorrelation of the noise, ##EQU8##

If the noise is white, then 3 dB improvement is obtained over the S/Nratio for received signal s(t) as shown in equation (10). Greaterimprovement occurs when the noise is positively correlated Since |_(n)(τ)|≦1, in no case can the performance be worse, even if the noise isnegatively correlated.

The improvement in noise performance is a result of using a matchedfilter matched to the original impulse pair signal shown in FIG. 3. Thedelay and subtract function is equivalent to a filter of impulse responeδ(t)-δ(t-T/2). This is a time shifted reverse of the input to thetransmit filter, δ(t-T/4)-δ(t+T/4).

INTERSYMBOL INTERFERENCE COMPARISON

The equation analogous to (6) or (7) for the combined signal y(t) usedby the present Manchester code receiver is ##EQU9## where

    h.sub.m (t)=g(t)-1/2g(t+T/2)-1/2g(t-T/2).                  (15b)

It is difficult to form general conclusions as to any reduction ofintersymbol interference by the present receiver of FIG. 1. However, itshould be noted that the interfering quantities given by (15b) consistsof the difference between samples and the average of samples a half-bittime before and after that sample. Since combined signal y(t) is sampledat approximately 503 and since signal s(t) is typically sampled at 403,it is apparent that the difference between these samples and therespective average of samples taken a half-bit time before and afterwould indicate that y(t) should exhibit a smaller intersymbolinterference characteristic than s(t).

The following description makes reference to FIGS. 7 and 8. The samplercircuit 706 operates in a similar manner to sampler 101 of FIG. 1 exceptthat two timing samples, x and y, are generated, respectively, at T1seconds after the delayed signal sample 801 and T/2-T1 seconds after thedelayed sample 801.

Auxiliary outputs of timing extractor 708 are generated to produce thesetiming intervals of T1and T/2-T1 after the signal sample time instantsgenerated by the timing extractor 708. In addition to the signalsampling time instants generated by timing extractor 708, auxiliaryinstants T1 and T/2-T1 occurring after those instants must also begenerated by the timing extractor 708. Note the signal sample 802represents the next signal sample of input signal 100. The timingsamples are outputted to a delay circuit which includes three delayunits, 701-703, in FIG. 7 whose delays sum to T/2. Note, the timingsamples and delayed timing samples are added by summer 103. However,decision circuit 105 ignores the timing sample sums and produces anoutput only in response to the signal samples 801 and 802.

The first delay 701 and last delay 703 are equal, and the total delay isequal to T/2. Illustratively, a good choice would be T1=T/8, T2=T/4.

As shown in FIG. 8, timing sample values, x and y, are on either side ofthe central zero crossing of the input signal 100. The timing extractor708 includes means for adjusting the sample time signal 707 so that theamplitude of timing samples x and y are equal and opposite.

The input signal 100 shown in FIG. 8 assumes that a logic 1 signal wastransmitted so that sample x is negative and sample y is positive.Summing circuit 704 sums samples x and y. Multiplier 705 multiplies theoutput of summer 704 by 1 when the input signal 100 is a logic 1. If alogic 0 signal is transmitted, the multiplier 705 inverts (i.e.,multiplies by -1) the sum so that the output signal to timing extractor708 is not dependent on whether a logic 0 or 1 is received.

As shown in FIG. 8, assume that the sampling time is too late. Then thesum x+y out of summer 704 will be positive, and the timing extractor 104will adjust the phase of sample time signal 707 so that the sampling canbe advanced. If the sampling time were early, then x+y would be negativeand timing extractor 104 would retard the phase of the sample timesignal 707. The sampling phase would be adjusted to the point wherex+y=0. When x+y=0, then sampling occurs at the central zero crossing ofinput signal 100. A phase locked loop could serve as the timingextractor 104.

FIG. 9 shows the system including a monitor, 901, which controlsselection circuit 902 to select either the input signal 100 or theoutput of multiplier 705 as the optimum input for timing extractorcircuit 104.

According to this aspect of the present invention, a performance monitorcircuit 901, responsive to a detected error rate or other transmissionimpairment measure in the receiver, may control selector circuit 902 toselect the signal to be connected to timing extractor 708. Monitorcircuit 901, illustratively, may monitor the noise error rate or othertransmission performance characteristic. Moreover, monitor circuit 901may also select the optimum delay in either or both the delay circuits701-703 and timing extractor circuit 104 to optimize transmissionperformance in a particular system.

What has been described is merely illustrative of the application of theprinciples of the present invention. Other methods and circuits can beimplemented by those skilled in the art without departing from thespirit and scope of the present invention.

What is claimed is:
 1. A Manchester code receiver comprisinginput means for receiving a Manchester encoded data signal, sampling means for generating samples of said data signal, delay means connected to said sampling means for delaying said data signal samples by a predetermined fraction of the time period of said data signal, and subtracter means for subtracting a delayed data sample outputted from said delay means from said data signal sample generated by said sampling means.
 2. The Manchester code receiver of claim 1 further comprisinga timing extractor means for selecting a sample time dependent on a zero crossing time of said received Manchester encoded data signal, and for controlling a sampling time of said sampling means.
 3. The Manchester code receiver of claim 2 wherein said sample time of said timing extractor means occurs approximately T/4 seconds before and after a zero crossing time of said data signal.
 4. The Manchester code receiver of claim 1 wherein said predetermined fraction is approximately T/2 seconds.
 5. A Manchester code receiver of claim 1 wherein said delay means is an analog sample and hold circuit.
 6. A Manchester code receiver of claim 1 wherein said sampling means generates a digital representation of said data signal, andsaid delay means includes digital storage means, and said subtracter means is a digital subtracter.
 7. The Manchester code receiver of claim 2 wherein said delay means includesa first delay circuit having a delay of T1 seconds, a second delay circuit connected to the output of said first delay circuit and having a delay of T2 seconds, a third delay circuit connected to the output of said second delay circuit and having a delay of T1 seconds, and wherein said timing extractor means includes means for combining the outputs of said first delay circuit and of said second delay circuit and means for adjusting said sample time in response to a signal received from said combining means.
 8. The Manchester code receiver of claim 7 wherein the sum of said delays T1, T2 and T1 is equal to approximately T/2 seconds.
 9. The Manchester code receiver of claim 7 whereinsaid timing extractor means selects a sample time dependent on a zero crossing time of a data signal connected to an input thereof, a monitor circuit for determining a data transmission performance parameter and outputting a control signal in response thereto, and selector means responsive to a first state of said control signal for connecting the input of said timing extractor means to said received encoded data signal and responsive to a second state of said control signal for connecting the input of said timing extractor means to an output signal from said combining means.
 10. A method of operating a Manchester code receiver comprising the steps of:receiving a Manchester encoded data signal, generating data samples of said data signal, delaying said data samples by a predetermined fraction of the time period of said data signal, and subtracting a delayed data sample resulting from said delaying step from next said data sample. 